I. Field of the Invention
This invention relates to sound reproduction structures and in particular to cabinet structures for loud speakers.
II. Description of the Prior Art
The conventional audio transducer or speaker comprises a conical metallic frame having at the apex thereof a permanent magnet. An electrically energized coil is forced to move with relationship to the permanent magnet as a result of current passing through the coil. This coil is connected to the apex of a conically shaped speaker cone which is movably coupled about the base thereof to the bottom section of the speaker frame. In this manner the speaker cone is driven back and forth with relationship to the permanent magnet and the frame of the speaker. Sound is radiated from the speaker cone as a result of the mechanical vibrations induced therein by the electro/mechanical transducer comprising the permanent magnet and coil. At the same time that sound is radiated from the speaker cone, additional vibrational energy is transmitted throughout the speaker frame. This vibrational energy is generally absorbed by a resiliant layer coupled between the speaker frame and the cabinet to which the speaker is attached.
The typical speaker cabinet comprises a top, a bottom and two side members together with a front and a back member. This combination of members forms a closed box, with the speakers generally being mounted on the front member for radiating their acoustical energy toward the listener. The typical speaker design requires a very firm attachment of edge sections of adjacent ones of the external speaker cabinet members in order to reduce the acoustical energy radiated therefrom as a result of the vibration of that member. The theory holds that only the speaker cone should radiate acoustical energy, since it is the only radiating element of the sound reproduction transducer which has its motion controlled by the electrical signal from the amplifier. Therefore, the typical speaker cabinet design requires various forms of internal support together with extensive edge reinforcement including edge-coupling adhesive substances for reducing the vibration of the external speaker cabinet members. In fact, some speaker enclosure designs have gone so far as to manufacture the external cabinet members of substances such as cement, earth, etc. which act as an infinite absorber and null radiator of acoustical energy.
In contrast with these prior art theories, the present invention recognizes that acoustical sounding boards are commonly used in other fine musical instruments such as guitars, violins, pianos, etc. These fine musical instruments are designed so that the prime source of acoustical energy, such as the strings of the piano, excite vibratile modes within the sounding board. These vibratile modes radiate additional acoustical energy which supplements and mellows the overall sound of the musical instrument as heard by the listener. This invention recognizes that if certain members of the speaker cabinet can be utilized as sounding boards, then the overall sound produced by the speaker together with its cabinet can be made more appealing in a psycho-acoustic sense.
This invention also recognizes that a significant portion of the electrical energy supplied to the electro-mechanical speaker transducer is dissipated in the form of vibrational energy produced by the movement of the cone of the speaker within the metalic frame. By coupling this vibrational energy from the speaker frame into the external members of the speaker cabinets, this normally dissipated vibrational energy, which is generally an accurate representation of the acoustical wave form produced by the speaker cone, is transmitted throughout the cabinet enclosure members and is thereby radiated as additional acoustic energy. This additional quantum of acoustical energy supplements the direct acoustical radiation from the speaker and increases the overall electrical to acoustical efficiency quotient of the system.
The prior art contains several examples of attempts by other inventors to utilize a vibratile sounding board to in some manner affect the tonal qualities of the speaker system. Hutchins in U.S. Pat. No. 3,233,695 discloses the use of a vibratile panel mounted internal to the speaker enclosure. This vibratile panel is designed to be driven substantially in phase with the speaker cone throughout a pre-selected low range of frequencies, thereby reinforcing the acoustical energy radiated over a specified frequency spectrum. This vibratile member is secured to the adjacent enclosure members by screws or other fastening means in conjunction with strips of foam rubber for dampening the edge motion of the vibratile panel. In this way the energy produced by the vibratile panel is not transferred to other elements of the speaker enclosure. The vibratile panel is designed to radiate only at low frequencies, and adds little to the high frequency qualities of the sound reproduction transducing system.
Klug in U.S. Pat. No. 3,109,509 discloses the use of a plurality of parellel transverse vibratile panels suspended within the speaker enclosure. These vibratile panels are coupled at one end thereof to the speaker cabinet by means of tensioned anchor springs. These vibratile panels coact with the walls of the speaker enclosure to accurately reproduce the recorded tones as well as amplifying the tones. The speaker cabinet, while formed of welded pieces of sheet metal, is of standard shape and construction.
Wenzel in U.S. Pat. No. 2,476,572 discloses the use of a plurality of vibratile panels which are independently suspended within the interior of the speaker enclosure. These vibratile panels reinforce the acoustical energy produced by the electro-magnetic speaker transducers in certain frequency ranges, predominately the bass or lower frequency portions thereof.
Drisi in U.S. Pat. No. 3,608,665 discloses the use of a plurality of speakers, each mounted on a planar surface of a pyramidal baffle. The rear end of the enclosure may be open to allow a rear directed radiation of acoustical energy from the electro-mechanical speaker element. This disclosure apparently does not utilize vibratile panels for modifying the tonal qualities of the acoustic spectrum generated by the system.
Other references which disclose concepts which are in part related to the theory of the present invention include the disclosures of Gerlach in U.S. Pat. No. 1,667,149, Duffy in U.S. Pat. No. 1,826,751, Brown in U.S. Pat. No. 2,058,407, Doubt in U.S. Pat. No. 2,602,860, Forrester in U.S. Pat. No. 2,646,852, Tavares in U.S. Pat. No. 2,704,185 and Eberhardt in U.S. Pat. No. 3,342,498. These prior art references add little to the references which have been previously discussed.
In contrast to the prior art references, the present invention utilizes a generally parallelopiped speaker enclosure having open back and front sections. The electro-magnetic speaker transducers are mounted within the rectangular void inside the enclosure for radiating their acoustical energy toward the front thereof. The vibrational energy produced by the motion of the speaker cone with reference to the speaker frame is transmitted through the speaker mounting elements to the external speaker cabinet members. Each of these external speaker cabinet members is allowed to vibrate in a periodic fashion which is sympathetic to the vibrational frequencies of the electro-magnetic speaker transducer. Furthermore, the edge sections of the adjacent external speaker cabinet members define therebetween isolation slots for allowing the maximum possible vibration excursion of the member in response to the energy transferred thereto. Nodal coupling blocks are utilized to transfer vibratile energy between adjacent ones of the external speaker cabinet members. The construction of these nodal coupling blocks will determine, at least in a general manner, the tonal characteristics produced by the sympathetic vibration of the external speaker cabinet members. These cabinet members may also be excited into their respectful vibratile modes by the incidence thereupon of acoustical energy produced by the electro-magnetic speaker transducer.